Can't really speak to where this sits on the infinite shelf of X-Men books, tie-ins, spin-offs, and crossovers, but I reallly liked it.
I had to read it with one hand on Wikipedia--there were all kinds of characters I'd never seen or heard of before (particularly w/r/t the New X-Men). So the first time through was slightly rocky but reading it a second time helped, I was able to just focus on the the story.
There are many different artists involved--some sections I liked better than others and some of the action scenes I found very confusing (it's clear "battle" is happening but it's hard to see who's doing what to whom and when in a few places).
But the storyline is just insanely good. The writers had a lot of disparate elements and lot of different storylines to converge, with sets of characters coming together and coming apart in different ways, and it was all handled masterfully. I also loved the original story outline they include at the end. SO INTERESTING and useful to anyone trying to plot stories like this--the parts where they were like "So uh we think somehow this has happened by now?" and you saw in the story how they solved that problem. Fascinating.
There are also some really memorable set pieces, the snow battle between X-23 and Lady Deathstrike being my favorite, but others too, Wolverine vs Predator X, the Cable/Bishop/Forge stuff in Act III.
I remember when I was a kid and the Mutant Massacre happened and I couldn't even believe they had written such a big story. And then M Day later on--I wasn't as into it by then, but it was clearly huge. And now to see where they have taken things since, it's pretty inspiring. There are all these cliches around comics, superheroes who keep coming back from the dead, things happening in alternate universes and timelines, on and on, but here a bunch of artists and writers worked on a project they were clearly passionate about, a huge storyline that impacts all kinds of characters and titles, and they did it successfully. Kudos, and kudos again.